1. Field of the Invention
The present invention relates generally to plating baths and methods for monitoring the major constituents contained therein. More particularly, the method of the present invention relates to a voltammetric analysis technique that provides signal spectra which accurately indicate concentrations of major constituents within the bath. The signal spectra can be used to maintain desired major constituent concentrations within limits in order to ensure optimal plating bath performance.
2. Description of Related Art
A typical plating bath solution is comprised of a combination of several different chemical constituents. The specific constituents vary depending upon the type of plating bath, but in general can be broadly divided into what are commonly known as major constituents and trace, or minor, constituents. The major constituents are defined as those chemical constituents which are in excess of 5 percent of the total bath volume. Trace or minor constituents, on the other hand, are defined as those present in smaller quantities, i.e. less than 5 percent of the total volume. For example, in an acid copper plating bath, a major constituent is sulfuric acid, which typically represents about 8 to 12 percent of the total volume. The acid copper plating bath might also contain trace constituents such as organic addition agents, degradation products and chemical contaminants, present in much smaller concentrations.
The concentration levels of both major and trace constituents are important determinants of the quality of the resultant plating deposit. Trace constituent concentrations influence certain characteristics of the plating deposit, including tensile strength, ductility, solderability, uniformity, brightness and resistance to thermal shock. Monitoring and optimization of trace constituents assumes that the major constituent concentrations within the bath are already properly set and maintained. Should the major constituents fall outside of required concentration ranges, however, the bath may fail to satisfactorily perform its plating function. It is therefore important that major constituent concentrations be regularly monitored.
Current techniques for monitoring the major constituents of plating baths typically involve removing a sample of the chemical solution from the plating tank for subsequent wet chemical analysis. Methods of measuring major constituent content in various types of plating baths are disclosed, for example, in K. E. Langford and J. E. Parker, "Analysis of Electroplating and Related Solutions", pp. 83-100, 65-68 and 174-180. In these analysis methods, for example, sulfuric acid content within an acid copper plating bath is determined using titration with sodium hydroxide; chromic acid content within a chromium plating bath is found using reduction titration with excess ferrous ammonium sulfate; free cyanide within a silver-cyanide plating bath is found by titration with silver nitrate; and carbonate within a silver-cyanide plating bath is analyzed by precipitation with barium chloride followed by titration with HCl. Major constituent concentrations in other types of plating baths are measured in a similar manner.
Wet chemical analysis methods such as the above must be performed by highly skilled personnel. Specialized and costly chemical analysis equipment and supplies are required. Furthermore, the delay between drawing samples and receiving measurement results can be anywhere from several hours to several days. It is thus very tedious and expensive to monitor major constituent concentrations using currently available techniques. Moreover, the slow response time of wet chemical analysis limits the extent to which a high quality and high speed plating bath can be continuously maintained.
The current major constituent monitoring techniques are quite different from real time trace constituent monitoring techniques such as those described in U.S. Pat. No. 4,631,116, assigned to the present assignee. The method disclosed therein uses voltammetric techniques to produce ac current spectra which vary as a result of changes in the concentration of various trace constituents. Voltammetric methods have been found to produce accurate results in real time for trace constituent analysis. However, voltammetric methods have not yet been considered for use in major constituent analysis. As a result, it is presently necessary to use voltammetric trace constituent measurement techniques in conjunction with the above-described major constituent wet chemical analysis in order to monitor the overall chemistry of the plating bath. The wet chemical analysis cannot be performed with the in-tank electrochemical sensors and other equipment typically used in trace constituent analysis. Two different sets of equipment must therefore be maintained in order to perform major and trace constituent analysis. No integrated measurement system is available which is capable of measuring both major and trace constituents.
As is apparent from the above, there presently is a need for an accurate and inexpensive real time method for monitoring the concentration of major constituents within a plating bath. Furthermore, the method should complement and be easily integrated with known techniques and equipment suitable for measuring trace constituents, resulting in an efficient overall plating bath analysis system.